on 12/8/04 2:33 PM, Ward, Ron (PWA) at email@example.com wrote:
December 3, 2004
Mark Brest van Kempen
Thank you for notifying the City of Oakland about the fluctuating flow levels of Chimes Creek in your backyard. As the Supervising Civil Engineer for the Public Works Agency, I am very interested in any impacts to Chimes Creek from the Leona Quarry development project, hereinafter referred to as the Project.
The Project is utilizing a detention basin to store storm water runoff and groundwater collected from the Project. The detention basin has the capacity to store approximately 2" of rainfall before overflowing into the storm drain system leading to Chimes Creek. None of the storms during the past month have overflowed the detention basin into the storm drain system.
Instead, the stored water is pumped into a settling and filtration system to clean the water of any sediments, and then released into the storm drain system and Chimes Creek. The pumping capacity is therefore controlling the discharge from the Project to Chimes Creek. The pumping capacity is approximately 350 gallons per minute, which is less than 10 percent of the flow of a moderate rainstorm prior to the construction of the project.
From your second e-mail, you noted that the creek flows were fluctuating in a three-inch deep concrete lined channel in your backyard whenever water was being pumped from the Project. This minor fluctuation in water depth should not be causing erosion to the creek banks. To the contrary, storing water from a short duration rainfall event and releasing it over several days has the effect of attenuating the creek flows reaching Chimes Creek from the Project. The peak flow volumes and velocity in Chimes Creek are reduced, while the flow duration of storm water is increased. Since erosion potential is increased by the squared factor of the flow velocity, the effect of pumping the stored water through a filtration system is a reduction in erosion potential when compared with the erosion potential before the Project began.
You mentioned the flocculent concentrations in the water coming from the Project. The contractor for the Project is using a Regional Water Quality Control Board approved flocculent known as "Chitosan". The concentrations used are approximately 0.5 ppm. Chitosan has been extensively tested for aquatic toxicity at a dosage of 1 ppm, and mortality rates of trout and daphnia minnows were insignificant. Furthermore, the water discharged from the Project is tested regularly for pH to confirm concentrations of Chitosan are well below 1 ppm.
The Project is expected to be pumping and filtering storm water from the site throughout the winter.
I hope this information provides assurance that Chimes Creek is being protected by reducing storm water runoff peak flows and stringent controls on sedimentation from the Leona Quarry development project. I can be reached at the phone number below with any questions you may have.
Supervising Civil Engineer